The present invention relates to a printing plate precursor and particularly to a printing plate precursor capable of forming an image by a computer to plate (CTP) system.
The printing plate precursor for CTP, which is inexpensive, can be easily handled, and has a printing ability comparable with that of a PS plate, is required accompanied with the digitization of printing data. Recently, many types of CTP by infrared laser recording have been proposed.
As one of these CTP, there is a process called as a wet type CTP in which solubility to a developer of an image forming layer of a printing plate precursor is changed by light exposure, followed by development to form an image. However, this process has various problems that an exclusive alkaline developer is necessary for development as in a conventional PS plate, developability is changed due to developer conditions (such as temperature and degree of fatigue), image reproduction is poor, and the handling ability in a lighted room is limited.
In view of the above, a dry CTP including development on a printing press has been developed, in which a specific developing process is not required. The printing plate precursor for dry CTP has been noticed since it can be applied to a printing press employing a so-called direct imaging (DI) process in which an image is formed on a printing press, followed by printing.
As one example of the dry CTP, there is an ablation type CTP, as disclosed in for example, Japanese Patent O.P.I. Publication Nos. 8-507727, 6-186750, 6-199064, 7-314934, 10-58636 and 10-244773.
These references disclose a printing plate precursor comprising a substrate and a hydrophilic layer or a lipophilic layer as an outermost layer. In the printing plate precursor having a hydrophilic layer as an outermost layer, the hydrophilic layer is imagewise exposed to imagwise ablate the hydrophilic layer, whereby the lipophilic layer is exposed to form image portions. However, there are problems that the ablation requires a high energy, resulting in a low sensitivity, and that the image formation due to physical breakage of the layer provides low dissolving power and a low dot quality.
Moreover, there are another problem that the contamination of the interior of the exposing apparatus by the scattered matter caused by the ablation of the surface layer. Accordingly, it is often necessary to have a built-in cleaning means such as a specific sucking device or a contamination preventing means such as to cover the surface of the printing plate element by a cover sheet, for solving such the problem. Furthermore, the process cannot be regarded to a complete dry process since it is necessary to remove the ablation residue remained on the plate surface by a means such as wiping or rinsing by an exclusive device.
In view of the above, a printing plate precursor, which is excellent in printing performance or a handling property, is required also as a printing plate precursor for an ablation type CTP.
On the other hand, a printing plate precursor, which is capable of forming an image without ablation and of requiring no development with a specific developer or wiping, has been proposed. and provides a good printing performance. There is a printing plate precursor for CTP capable of being developed (development on press) with dampening water on a printing press disclosed in, for example, U.S. Pat. Nos. 2,938,397 and 2,938,398, which comprises an image forming layer containing thermoplastic polymer particles and a water soluble binder. However, such a planographic printing plate, when a grained aluminum plate is used as a hydrophilic substrate, is required to contain a light heat conversion material (generally colored) in the hydrophilic layer, and may contaminate a printing press in development on press.
As a method to prevent the printing press contamination in development on press, there has been proposed a method employing a printing plate precursor comprising a substrate having a hydrophilic layer containing a light heat conversion material. This hydrophilic layer can eliminate a light heat conversion material from an image forming layer, but it is difficult to give such a substrate having the hydrophilic layer to a printing performance equal to the grained aluminum substrate. Many proposals have been made, but those having a sufficient printing performance have not yet been obtained.
For example, Japanese Patent O.P.I. Publication No. 2000-355178 discloses a hydrophilic layer containing a hydrophilic light heat conversion material. However, hydrophilicity of other materials contained in the hydrophilic layer is not taken into consideration, and in the examples, no contamination is produced when printing is carried out employing dampening water containing isopropanol in an amount of 10% by volume. This hydrophilic layer is not suitable for printing carried out employing dampening water containing no isopropanol, which is required at present for improving working environment.
Further, change in nature of the light heat conversion material contained in the hydrophilic layer, which occurs on image formation employing heat generated by the light heat conversion material subjected to infrared laser exposure, is not considered, also. The change in nature of the light heat conversion material itself (such as oxidation, decoloration or vaporization) may lower strength of the hydrophilic layer at image portions after image formation, which may lead to the problems that in printing, images or the hydrophilic layer are removed.
A first object of the invention is to provide a printing plate precursor having a hydrophilic layer which is applied to CTP requiring no specific development, and provides a good printing performance. A second object of the invention is to provide a printing plate precursor having a hydrophilic layer containing a light heat conversion material, in which when subjected to infrared laser exposure, an image can be formed without ablation. A third object of the invention is to provide a printing plate precursor having a hydrophilic layer, in which after subjected to infrared laser exposure and heated, the strength of the hydrophilic layer is not lowered. A fourth object of the invention is to provide a printing method employing the printing plate precursor described above, which provides a good working environment.